Quote:
Originally Posted by gregsfc
I'm currently running 10W40 Castrol-brand, semi-synthetic and getting better mpg in warm weather than I got when I had 10W30 Amsoil, full synthetic in it. But one thing has changed that seems to have a much greater effect than oil viscosity and that's a different brand of tires; going from Bridgestone Battlax's to Michelin P4s; the latter creates a more optimistic trip meter error, but even after accounting for that error, I'm getting one or two more mpg for commutes and at least two better for trips.(SNIP)
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Typically, that would not be the case.
Thinner oils will usually give better mileage. An oil with a low "W" rating usually helps also, because the oil is thinner during the time it takes to reach its full high temp rating. That amount of time can be significant, so the thinner oil can show improvements. Synthetic oils typically flow better at all temps than conventional oils and give better mileage.
A 5W-30 synthetic cycle oil should give you better mileage than a 10W-40 or a even a 10W-30 of any type.